Repotting is a task which needs to be performed on all bonsai periodically. However rather than following a prescribed frequency you need to be able to determine whether a bonsai actually needs to be repotted or not, because it is does initially seriously weaken the tree.
The vigor of the tree is down and it has been several years since you last repotted. Repotting a tree gives it a boost of energy, assuming of course it recovers from the initial shock of repotting.
Water does not drain from the container and oxygen penetration is impeded. This is likely as the growing medium has compacted and has essentially become a solid mass or simply as the container has filled with roots.
Root rot is a disease which can infect a variety of trees particularly conifers (junipers, pines, cypress etc) and olives by attacking their roots and it is caused by incorrect watering or growing medium which is too water retentive (or a combination of both) essentially drowning the roots.